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Community and Q&A

Exterior wall insulation

RFoll | Posted in General Questions on

Im looking for opinions on which method would be best for my area (4). Since I don’t want to build 12″ double exterior walls, I’m planning to use 2X6 with packed cellulose and either 2″ of styrofoam exterior sheeting or zip sheeting with rock wool exterior insulation over that. Also I think I want to use rigid foam vents furring the rafters down and packing cellulose to an acceptable r factor for a conditioned attic. As you can see I’m completely new to green building and not looking to build a passive house but one that will work good for my area, thanks for reading your opinion is important to me.

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  1. Stockwell | | #1

    Where will you be building Ralph?

  2. RFoll | | #2

    The new home will be in middle Tennessee

  3. GBA Editor
    Martin Holladay | | #3

    You may want to download a free energy modeling program like BeOpt so that you can use the program to refine the design of your house.

    1. In your climate zone, you may not see a large energy return from exterior rigid foam or exterior mineral wool. A well-built 2x6 wall may serve your needs.

    2. Paying attention to airtightness will yield an excellent return in energy savings. Set an airtightness target and use a blower door during construction to help you meet your goal.

    3. Here is a link to an article that describes ways to create an insulated sloped roof assembly: How to Build an Insulated Cathedral Ceiling.

  4. Expert Member
    Dana Dorsett | | #4

    In US climate zone 4A (most of TN) the IRC 2015 code minimum is 2x6/R20 or 2x4/R13 + R5 continuous insulation.

    A cheaper way to beat code-min at a standard 2x6 wall thickness would be 2x4 framing + 2" of insulating edge strips, made of 2" ZIP-R ripped down to 1.5" strip to create a 5.5" cavity (or 1.5" of polyiso + 1.5" CDX plywood glued together), with the foam side of the ZIP R glued to the stud edges, then insulating with 5.5" of cellulose. The strips can be both glued & nailed into place, and the wallboard gets fastened to the strip wood. The 1.5" of polyiso + 0.5" OSB/CDX brings the R-value of the framing fraction up from about R4 to about R14 which improves the whole-wall performance up near to the proposed 2x6/20 + 2" EPS solution at a fraction of the cost.

  5. RFoll | | #5

    Thanks for you comments Martin, still thinking of at least 1" rigid foam for a thermobreak. Dana I can see where your suggestion would save me money but wondering how you handle wall outlets. Is it possible to fasten them to 1/2" usb/cdx, does the zip-r work as a thermobreak fastened to the interior side of the wall?

  6. GBA Editor
    Martin Holladay | | #6

    One thing I've learned over the years: everyone has a different opinion on wall details, and lots of different approaches can work. In general, it's best to choose the approach that makes sense to you (and, of course, your builder, assuming you're not doing the work yourself).

    If you want to install a continuous exterior layer of rigid foam, by all means, do so.

    When it comes to electrical boxes, manufacturers have come up with 20 different variations on boxes, all with different attachment methods. Visit a good supply house and you can choose an electrical box that works for you.

  7. Expert Member
    Dana Dorsett | | #7

    A 2x6/R20 wall with an inch of continuous exterior polyiso works fine from a moisture resilience point of view in zone 4A, and would beat IRC code minimum values even in zones 7 & 8 (= R20 cavity + R5 continuous insulation minimum).

    The IRC doesn't even prescribe a minimum exterior R for 2x4 or 2x6 framing for zone 4A, but for zone 4C (which has a shorter & shallower drying season than 4A) it specifies R3.75 minimum. With an inch of foil faced polyiso you'll be fine. See:

  8. RFoll | | #8

    Thanks for your answers, I have been building homes for many years and always built to the local code. I have always wanted to do a better job. So at 73 yrs old I have decided to build a better home despite what my customers want. So I'm sure I'll be back with other questions, thanks.

  9. RFoll | | #9

    Talked with a foam insulation contractor today and wanting to pass on his advise. I want to foam or insulate my roof to the point I have a conditioned attic. His advise was to blow 6" of open cell foam on every surface of the roof. I asked him what R factor 6" of foam would give me. His answer was R-20 and would pass the required local building and provide a complete air barrier for the roof. In the members opinion here is that enough or am I just meeting minimum building code again? His cost is $1.65/ft for roof and $1.50/ft for for foundation walls and encapsulating the crawl space. Yes I'm building over a crawl space the benefits down the road are greater in my opinion then the drawbacks. Again thanks for reading and passing along your advise.

  10. GBA Editor
    Martin Holladay | | #10

    In Climate Zone 4, the 2012 code requires a minimum of R-49 roof insulation, not R-20. Your contractor is wrong.

    This is a common problem with spray foam contractors. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.

  11. Expert Member
    Dana Dorsett | | #11

    Note: 6" is the maximum depth most open cell foam products that can be safely installed in a single pass. Going deeper requires a cooling off period during curing, which is both a quality and fire-hazard (during curing) issue. Foam installers almost magically come up with reasons why 6" is exactly the right amount. But behind the raionale are the facts that the quoted cost of 6" is less likely to scare the clients away than full code compliance, and 6" is the most profitable for them, since it doesn't require tying up a truck & crew waiting for the first pass to cure/cool.

    In zone 4 you can hit code on a U-factor basis with 6" of open cell foam on the underside of the roof deck AND 4" of rigid roofing polyiso above the roof deck (R22-ish) . If you can find a source for reclaimed roofing foam it's typically 1/4 to 1/3 the cost of virgin stock goods,

  12. RFoll | | #12

    Wow Martin that information was a mouth full. Thanks to both you and Dana for your responses. So I'm off foam in the rafters. My Daughters home was completely foamed and performs poorly compared to a home I presently live in, I have wondered what was wrong with it now I know. Foaming the house was my son-in-laws input. Shame I didn't recognize the flaws in the beginning.
    So now I'm talking to my regular insulation contractor about full vents against the bottom of the decking, furring down and using 10" high density batts and then covering with a air barrier of some type (tyvev or sheetrock). I asked him if I could still insulate between floors and attic ceiling as I have in the past. He felt it would not be a problem. All this would require a lot of extra labor but I'm more comfortable trying to achieve a better home by this method. As always looking forward to your comments, thanks. Ralph

  13. GBA Editor
    Martin Holladay | | #13

    If you want more information on all of the different ways to create an insulated sloped roof assembly, you should read these two articles:

    How to Build an Insulated Cathedral Ceiling

    Creating a Conditioned Attic

    Before you go forward with your plan to use plastic housewrap as an interior air barrier, you should check with your local code enforcement authority. As far as I know, plastic housewrap hasn't been approved for interior use (due to fire safety concerns). I may be wrong on this point, so check with your inspector.

  14. RFoll | | #14

    OK I read the articles you suggested so my next idea is as follows. Use 10" I beam for rafters, install job site air baffles, 8"+ insulation (batts or dense pack) and 2" foil face foam board. If this is workable do I assume the air barrier should be at the foil face and the seams taped? Also thinking of stopping 10' above attic floor and sheetrock the ceiling and having a small vented attic with R-50+ or as close as I can get. I'm just really trying to get my H&A in a conditioned area of the house. Again thanks for your time in helping me seems the more I read the less I'm sure what to do.

  15. GBA Editor
    Martin Holladay | | #15

    Your plan will work. Some jurisdictions allow foam board to be exposed in attics if access is limited -- but other jurisdictions will insist that the foam board be protected with drywall for fire safety, regardless of what the attic is used for. Call up your local building department for more information.

    If the attic has stairs, or a finished room, you definitely need drywall.

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